Look-up tables are tables that are connected to other tables as drop-downs to make data entry easier. This is very convenient for oft-repeated data entry areas, such as country or say 'productcategory'. Enabling users to select data from a drop is great for data integrity, increasing accuracy and reducing spelling errors.

In this example, FurColor is a look-up table:

TeddyBears
Name: Fuzzy
Nickname:
FurColor: BrownWhiteBlueBlackBeige

I could only have this drop-down if I already have a FurColor table in my application.

See also Creating a Look-Up Table, Adding a Drop-Down Field, and Convert a Field to a Drop-Down.